1. Field of the Invention
This invention relates to a method for manufacturing a thermal printer head (thermal head).
2. Background Art
A thermal head includes a resistance heating element and electrodes forming one circuit set multiply disposed in substantially straight line configurations on an insulating substrate made of ceramics, etc. A protective layer made of an insulative material is formed to cover at least the series of resistance heating elements. Printing is performed by providing pulse currents to the resistance heating elements to generate heat in a state where printing paper is pressed onto the upper face of the protective layer via an ink ribbon or in a state where thermal paper is pressed onto the upper face of the protective layer.
The protective layer may be formed by thin film formation methods typified by sputtering and thick film formation methods (film coating methods) typified by screen printing. Although protective layers formed by thin film formation methods have high heat resistance and thermal heads using such protective layers have excellent printing performance and durability, manufacturing costs are undesirably high. Therefore, the practical development of protective layers formed by film coating methods is expected to reduce costs. In film coating methods, a glass paste having glass as the main component is coated as a film, and sintering is subsequently performed to obtain a glass protective layer. Further, a corrosion prevention layer made of oxides, etc., may be provided between the glass protective layer and the resistance heating element and electrodes to prevent the resistance heating element and the electrodes from corroding during the sintering of the glass protective layer.
On the other hand, the resistance heating elements of the thermal head are multiply provided corresponding to the number of printed dots. In the case where the electrical resistance of the multiple resistance heating elements fluctuates, the amount of heat generated by each of the resistance heating elements is different, resulting in uneven optical density. To solve such problems, the resistance values of the resistance heating elements of the thermal head may be adjusted to prescribed values (JP-A 4-8555 (Kokai) (1992)). In such a method, a pulse voltage is applied to each of the resistance heating elements to change the electrical resistance value to the prescribed value. Generally, this resistance value adjustment (also referred to as “bit trimming”) is performed after completion of the thermal head, that is, after forming the protective layer. Although the resistance value adjustment can be performed without problems in the case where the protective layer of the thermal head is formed by the thin film formation method providing a high heat resistance, the glass protective layer formed by the film coating method recited above undesirably melts due to heat emitted by the resistance heating elements during the resistance value adjustment.
Methods forming the glass protective layer by the film coating method after adjusting the resistance values may be considered. Although the electrical resistance of the resistance heating elements can be adjusted to the prescribed value by the resistance value adjustment in such a case, the electrical resistance undesirably varies due to the subsequent sintering at a high temperature during the glass protective layer formation process, and practical problems remain.